A Model for Aircraft Aging

An aircraft is a complex machine composed of many interrelated components and systems. When these electrical and mechanical systems are designed there is an expected life length. As the aircraft and systems age they gradually degenerate until they are no longer able to perform the functions for which they were designed. When a component or system is not functional it can be replaced or repaired and the condition of the aircraft is restored to some degree. With age and fatigue the frequency of repairs and maintenance costs increase until it is more cost effective to replace the aircraft with a new one. Although replacement time was set at 20 years for many aircraft models, this has not happened. It has been assumed that the operating condition of aircraft can be maintained at a high level beyond the intended life through maintenance. However, the degeneration with age inevitably occurs, although it is modified by a number of factors (Alfred and Thombs, 2002): quality of repair work; intensity of use; deferral of planned maintenance. In addition to the possibility of maintaining a high operating condition, there are other reasons for not retiring an aircraft from the fleet: the high cost of new aircraft; the increase in demand, which requires an expanded fleet; earlier shortage of production capacity for new planes. (Friend, 1992). The result of these and other factors is a large number of aircraft in use beyond their planned retirement. In 1997 there was 46% of US commercial aircraft over 17 years of age and 28% over 20 years. It could be claimed that commercial aircraft are being taxed to perform well beyond their intended operating life. Of course if this is true it would be expected that either the rate of aircraft failure would show a corresponding increase or the operating hours would decrease since planes would be out of service for repairs more frequently. In this paper the aging hypothesis: increasing failure rate with age, will be considered with data from Service Difficulty Reports (SDRs) and aircraft use (cycles). The analysis in this paper eliminates many confounding variables by focusing on a single aircraft model and a single carrier. Furthermore, the only failure considered is an unscheduled landing due to mechanical problems affecting safety, an outcome that is clearly defined. The results are specific to the experimental conditions of that study group. However, the age factor can be analyzed without the complications arising from variable reporting, variable repair practices, and mixed fleets for instance. In section 2 of this paper, aging concepts are developed, which provide a theoretical framework for a Poisson regression model for the number of failures with the failure rate depending on age. The database used to estimate the parameters in the model is discussed in Section 3. The results of model fitting are presented in Section 3. The data provide clear evidence that the number of unscheduled landings conform to the pattern described by the model with a failure rate depending on age.